The structure of yeast nuclear pore complexes is being studied both by cryo-electron microscopy and 3-D image reconstruction and the STEM. STEM mass analysis of the complexes is being used to place volume constraints on 3-D maps being calculated using the random conical tilt method. The mass analysis will also place firm limits for future work on the stoichiometry of nucleoporins. Preliminary mass measurements give an estimate of 65-68 Md for an intact complex, but clearly some specimen disassembly is occurring during specimen preparation. Studies are underway to try to improve specimen preservation including fixation.